This invention relates to a gamma ray scattering or gamma-gamma density well logging technique which is carried out in the presence of background radiation, and more particularly, to techniques for determining formation bulk density notwithstanding the background radiation. The term background radiation refers to radiation naturally occurring in earth formation in the vicinity of a well borehole, and typically is formed by the decay of potassium, uranium or thorium and their daughter decay products (referred to herein as K-U-T elements). In formations rich in these elements the naturally occurring background can be substantial.
This disclosure is directed to well logging tools to be used in either old wells or in new wells. It is often necessary to use nuclear measurement techniques to measure formation characteristics. This is true of both old and new wells, new wells including wells which have been drilled but not necessarily cemented and cased. An attractive technique is the gamma ray scattering approach, sometimes called gamma-gamma density measurement of formation bulk density. The bulk density is derived from the scattering characteristics of the formation for relatively low energy gamma radiation emitted from a radiation source carried in a downhole sonde. In gamma-gamma logging, a gamma ray source is utilized to irradiate formations adjacent to the well borehole. This may occur in an open borehole (one with no casing and cement) or may occur in an older well having various thicknesses of casing. The formation is irradiated from the source and scatters gamma rays impinging on it which are detected by a pair of spaced gamma ray detectors which produce count rate signals. The signals are interpreted in terms of electron density or formation bulk density of the various strata or earth's formations adjacent to the borehole. The measurement of bulk density is used to determine other formation parameters of interest. As an example, bulk density along with the response of a neutron log can be used to identify gas saturated formations. Recently issued U.S. Pat. No. 4,297,575 sets forth a procedure whereby bulk density can be determined, and thickness of the casing in a cased hole is also indicated. This is accomplished in the presence of minimal background radiation. In some situations, the well borehole may pass through sufficient concentrations of the K-U-T natural gamma radiation to experience sufficient K-U-T background radiation that the data which would otherwise be obtained is obscured. Because the K-U-T radioactive isotopes have a relatively long half-life, if they are found anywhere in the near vicinity of the borehole, they can pose a background radiation problem.
This invention sets forth a new, useful and unobvious technique which yields borehole compensated density measurements corrected for the effects of naturally occurring gamma radiation. The technique utilizes only two detectors. One of the detectors is operated to obtain a gamma ray energy spectrum. The spectrum is divided into four energy windows. Once corrections are made to remove the effect of the naturally occurring gamma radiation in the second detector, the bulk density and density correction are computed utilizing a "spine and rib" procedure, thereby obtaining density independent of gamma ray impingement from the formation of interest. As a by product, this technique also yields K-U-T elemental concentrations.